The present invention relates to an exhaust emission control system of an engine, and particularly to an exhaust emission control system which is provided in an exhaust passage with a NOx catalyst which purifies NOx in exhaust gas.
Conventionally, exhaust emission control systems of engines, which include a selective catalytic reduction (SCR) catalyst disposed in an exhaust passage of the engine and for purifying NOx within exhaust gas by causing a reaction with NH3 and a NOx storage catalyst which stores (occludes) NOx contained in exhaust gas when an air-fuel ratio of the exhaust gas is lean (i.e., λ>1, larger than a theoretical air-fuel ratio) are known (e.g., see JP3518398B). Such NOx storage catalyst further reduces the stored NOx when the air-fuel ratio is approximately equal to stoichiometric (i.e., λ≈1, approximately equal to the theoretical air-fuel ratio) or is rich (i.e., λ<1, smaller than the theoretical air-fuel ratio). In the exhaust emission control system of the engine, the SCR catalyst purifies NOx when an engine speed and an engine load are high, i.e., the temperature of the SCR catalyst is high, and otherwise the NOx catalyst purifies NOx.
Further, JP2010-112345A discloses an art for performing NOx purification with an SCR catalyst by adsorbing NH3 generated in a NOx reduction control of a NOx catalyst, instead of providing a urea injector for injecting urea to the SCR catalyst.
With the art of JP3518398B, if a urea injection by a urea injector is not performed normally within an operating range of the engine where the SCR catalyst purifies NOx, i.e., the NOx catalyst does not purify NOx, the NOx purification by the SCR catalyst becomes insufficient, and a problem arises that a large amount of NOx is discharged.
Therefore, as described in JP2010-112345A, NH3 generated in the NOx reduction control of the NOx catalyst may be supplied to the SCR catalyst.
However, the amount of NH3 generated in the NOx reduction control by the NOx catalyst is relatively small, and NH3 supplied to the SCR catalyst is not enough to sufficiently purify NOx, thus a problem arises that the SCR catalyst cannot sufficiently purify NOx.